The present disclosure relates to drag reduction devices for land vehicles, such as trucks and tractor-trailer rigs.
Fuel economy is a persistent concern for all land vehicles and is particularly acute for large vehicles such as trucks and tractor-trailer rigs. Fuel economy improvements have been achieved by innovation in engine design and improvements in fuel composition. However, the size and shape of the vehicles plays a substantial role in fuel economy. Ultimately, drag is the greatest enemy to fuel economy, with as much as 70% of the engine power devoted to cutting through the air in front of the vehicle.
Drag is a force that resists the movement of a body through a fluid, whether the body is a baseball and the fluid is air, or the body is a swimmer moving through water. Drag is a function of two components—friction drag and pressure drag. Friction drag is a force that acts tangential to a surface of the body. Friction drag is increased by irregularities or roughness on the surface and decreased by making the surface more slippery. A clean truck cuts through the air more efficiently and with less friction drag than a dirty truck.
Pressure drag is a force that acts perpendicular to a surface. Pressure drag is a function of the surface area in the direction of travel as well as the velocity or speed at which the body is traveling. Pressure drag increases as the square of velocity so that doubling vehicle speed actually creates four times more pressure drag. On the other hand, pressure drag is directly related to surface area so that a ten percent reduction in surface area leads to a ten percent decrease in pressure drag.
For aerodynamically configured vehicles, such as airplanes, friction drag contributes more heavily to overall drag than pressure drag. However, for land vehicles, such as a typical tractor-trailer, pressure drag can be as much as ten times greater than friction drag due to the large frontal surface area of the truck. Unfortunately, the size of these types of vehicles is dictated by their function—hauling products or materials. Unlike passenger vehicles, the box-like shape of trucks cannot be significantly altered. A smaller frontal surface area means a smaller truck which means less cargo that can be hauled. Pressure drag in land vehicles, and especially in trucks, is increased by pressure “hot spots”, such as beneath the undercarriage, behind the rear of the trailer or between the tractor and the trailer. These hot spots are generally regions of low pressure which causes air flowing over the vehicle to deviate from a streamlined path around the vehicle. Vortices can form in these hot spots that significantly increase the pressure drag.
In quantitative terms, if a square body has a drag coefficient (CD) of 1.00, elongating the body into a rectangular shape reduces CD to 0.80. Adding a rounded nose cuts the coefficient in half to 0.40. Adding a “boat tail” or a conical tail decreases CD further to 0.20. It has been estimated that a 20% reduction in drag yields at least a 10% increase in fuel economy at highway speeds. For truckers and trucking companies, this increase in fuel economy means significantly reduced fuel costs year in and year out. For the environment, increases in fuel economy mean fewer deleterious emissions. A significant amount of effort has been expended in developing drag reduction technology for trucks. These efforts include streamlining the tractor, introducing seals, air deflectors or vortex generators in the gap between the tractor and trailer, and adding undercarriage skirts, guide vanes, air deflectors and boat tails to the trailer. Each of these modifications contributes in some measure to the overall drag reduction, so a fully optimized rig will incorporate a number of these improvements.
However, all of these efforts have been directed to the tractor or trailer itself, and not to accessories associated with the vehicle. One common accessory is the underbody storage box, an example of which is depicted in FIG. 1. The storage box S is mounted to the frame F of the trailer T, either in front of the rear wheels W or in some cases between tandem rear wheels. The typical underbody storage box is 24 in. tall, 24 in. deep and 60 in. long. Not only does the square face of the storage box increase the overall pressure drag of the trailer, the storage box can also increase the turbulence of the airflow beneath the trailer. Not only does this increased turbulence add to the drag, it can also lead to increased road spray lateral to the trailer when traveling on wet or slushy roads.
Consequently, there is a need for making the underbody storage box more aerodynamic to reduce the impact of the storage box on the overall drag of a trailer.